24 in telescope - completion of mirror cell

自製天文望遠鏡 牛頓望遠鏡 杜蘇式 卡式 設計和磨鏡方法<br>
佛科試鏡法 佛科刀片儀 光柵試鏡法 光柵片
回覆文章
Chanlunlun
夸克星
文章: 3800
註冊時間: 週四 09 10月, 2003 21:06

24 in telescope - completion of mirror cell

文章 Chanlunlun » 週五 09 4月, 2004 09:06

Dear ATMs,

The 18 points floatation cell for the 24 inch mirror is completed today. Upon completion the cell was immediately tested by putting the mirror onto it. The mirror was resting on it stable.

Of course the mirror must not deform while it is on the cell, so another test was carried out – the cell, with the mirror resting on it, was taken onto an open area and tilted at approximately 60 degrees with the ground, targeting on wall tiles of a distant building. A Meade Ultra Wide 8.8 mm eyepiece ( giving 240X ), hand held against a tripod, was used to view directly into the mirror ( the area of the mirror is so large that my head practically did not block off much light ). If there was deformation of any degree on the mirror, however slight, an astigmatic image will be observed easily.

The result – a sharp and clean image was observed, even at 240X, showing that the mirror cell is good. No apparent deformation of the mirror was observed.

For telescope equipped with ultra thin fast mirror, the role of the mirror supporting cell is of vital importance. Because of the deep curvature of the mirror the distribution of mass is very uneven along the diameter and this call for the complex mathematical method called finite element analysis to decide where to best put the 18 supporting points, in order to minimize mirror deformation. The technique is, unfortunately, hopelessly complicated to handle with and hence it was impossible for us to design a mirror cell for an ultra thin fast big mirror even a few years ago.

Fortunately with the abundance of resources for ATMing on the web nowadays the computation for mirror cell can easily be done with free software within seconds. Our mirror cell is designed by finite element analysis software downloaded from the web. This again shows how information technology can assist in telescope making .

Computation shows that for best result we should use 27 points instead of 18 points to support the mirror. Nevertheless analysis also showed that an 18 point cell, with optimized location of supporting, still gives deformation less than 1/8 wave. Since the fabrication of a 27 points cell is exceedingly complicated and we have no experience on it so we have chosen the 18 points cell.

Although the finished mirror cell looks a little bit ugly, it consists of metal parts precisely machined. It was carefully fabricated where it needed to be.

With the completion of the primary mirror cell the fabrication of the secondary cage will be followed. The star test will be carried out not until the whole telescope is finished.

Best regards

Chan Yuk Lun
8-4-2004
附加檔案
contour.gif
Calculated deformation of our mirror under our 18 points cell. Blue color represents +1/13 wave deformed, dark green : +1/25 wave, bright green : +1/60 wave, red : -1/21 wave. Hence P.V.deformation is about 1/8 wave.
contour.gif (33.88 KiB) 已瀏覽 9734 次
mirror_cell_with_mirror.jpg
The mirror cell with mirror resting on it. At bottom of diagram is an12.5 mm eyepiece, just to give you an idea the size of the mirror and cell.
mirror_cell_with_mirror.jpg (70.27 KiB) 已瀏覽 9734 次

頭像
Wah!
夸克星
文章: 9073
註冊時間: 週六 28 6月, 2003 01:19
聯繫:

文章 Wah! » 週五 09 4月, 2004 09:31

好大件, 支點用軟膠會唔會好些?

頭像
天屍仔
原恆星
文章: 65
註冊時間: 週六 12 7月, 2003 20:14

文章 天屍仔 » 週五 09 4月, 2004 10:40

Wah! 寫:好大件, 支點用軟膠會唔會好些?
why gei?

Chanlunlun
夸克星
文章: 3800
註冊時間: 週四 09 10月, 2003 21:06

文章 Chanlunlun » 週五 09 4月, 2004 11:51

Wah! 寫:好大件, 支點用軟膠會唔會好些?
Dear Mr. Wah!,

In order to facilitate perfect point of contact the contact point must be as hard as possible, and the contact area must be as small as possibe. Hence a best contact point so be a bronze ball head, compromising between price and effectiveness.

It is not possible to use soft material to make contact point because

1. There will not longer be any point contact, instead the contact point smears into an area, in which case the noraml reactional force cannot be predicted.

2. Soft material produces friction, which again upset the delicate balance of reactional forces on all contact points.

Above two effects are very small indeed, but in big mirror such as ours, these small effect accumulates and cause disastrous result.

For mirrors smaller than 16 inch aperture, we no longer need true floatation mirror cell, all we need is just to tape the back of the mirror by foam tape, usually we choose 9 points. One thing we need to remember - the back plate must be PERFECTLY FLAT otherwise the mirror still deforms.

Best regards

Chan Yuk Lun

頭像
alyf
主序星
文章: 174
註冊時間: 週日 29 6月, 2003 21:58

文章 alyf » 週五 09 4月, 2004 19:22

chan sir,
when will u carry on the work in easter? I also want to come back and have a look.....

頭像
Wah!
夸克星
文章: 9073
註冊時間: 週六 28 6月, 2003 01:19
聯繫:

文章 Wah! » 週五 09 4月, 2004 21:11

Chanlunlun 寫: 1. There will not longer be any point contact, instead the contact point smears into an area, in which case the noraml reactional force cannot be predicted.

2. Soft material produces friction, which again upset the delicate balance of reactional forces on all contact points.

Above two effects are very small indeed, but in big mirror such as ours, these small effect accumulates and cause disastrous result.
How serious will these effects be on your 24" mirror?

I think soft material can decrease the pressure on the contact points, which will not bend the glass as much as using a small pin point.(Not effidence, but just I think.)

Chanlunlun
夸克星
文章: 3800
註冊時間: 週四 09 10月, 2003 21:06

文章 Chanlunlun » 週六 10 4月, 2004 23:15

alyf 寫:chan sir,
when will u carry on the work in easter? I also want to come back and have a look.....
Dear Mr. alyf,

You may come on Monmday dusk, if you don't mind.

Best regards

Chan Yuk Lun

回覆文章

回到「自製天文望遠鏡 Amateur Telescope Making 」

誰在線上

正在瀏覽這個版面的使用者:沒有註冊會員 和 0 位訪客